1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus, multiplex machine or similar image forming apparatus. More particularly, the present invention relates to a driving device included in, e.g., an image forming apparatus for driving a plurality of image carriers or rotary bodies.
2. Description of the Background Art
A driving device of the type transmitting the output torque of a drive source to a driven member via a gear is conventional. It is a common practice with this type of driving device to arrange idler gears in a gear train in order to drive a plurality of driven members with a small number of drive sources.
An image forming apparatus belongs to a family of apparatuses using a plurality of driven members. A color printer or similar image forming apparatus, among others, uses a plurality of photoconductive elements or image carriers for forming a full-color image. One of conventional color printers includes image forming units arranged side by side and each being capable of forming a toner image of a particular color on a respective photoconductive element. In this type of color printer or tandem color printer, toner images formed by the image forming units are sequentially transferred to an intermediate image transfer body one above the other, completing a full-color image on the intermediate image transfer body. The full-color image is then transferred from the intermediate image transfer body to a sheet or similar recording medium. Another type of tandem color printer is constructed to convey a sheet via consecutive image forming units while sequentially transferring toner images formed by the image forming units to the sheet one above the other, thereby forming a full-color image on the sheet.
In the tandem color printer, the photoconductive elements included in the image forming units are rotated in the same direction as each other to transfer toner images to the intermediate image transfer body or the sheet. The photoconductive elements each are assigned to one of four colors, i.e., yellow, cyan, magenta and yellow complementary to separated colors.
The photoconductive elements of the image forming units each may be driven by a respective drive source or may share a single drive source, as well known in the art. In a drive system using a single drive source, a gear is mounted on the shaft of one photoconductive element, which is directly driven by the drive source, while an idle gear is held in mesh with the gear, so that the rotation of the one photoconductive element is transferred to the other photoconductive elements via the driven gear and idle gear. A problem with this type of drive system is that any eccentricity or irregularity in diameter of each photoconductive element, driven gear, drive gear or idler gear causes the rotation speed of the photoconductive element to noticeably vary, resulting in banding or image shift. Although this problem may be solved by a scheme capable of reducing eccentricity or irregularity in diameter, such a scheme makes production difficult and increases cost.
To reduce the mutual influence of the irregular rotations of the photoconductive elements, Japanese Patent No. 3,107,259, for example, discloses a drive system in which a rotary encoder is mounted on a shaft driven by a motor for driving a photoconductive element. Feedback control or feedforward control is executed with the motor in accordance with a phase signal output from the rotary encoder such that the rotation phases of the photoconductive elements are matched to each other. Also, Japanese Patent Laid-Open Publication No. 6-167858, for example, teaches a system in which the reduction ratio of idle gears intervening between photoconductive elements is increased to obstruct the transfer of a phase shift from one photoconductive element to the next photoconductive element.
However, U.S. Pat. No. 3,107,259 mentioned above has a problem that an exclusive drive source must be assigned to each photoconductive element, and moreover arrangements for monitoring the rotation speed of the individual drive source is essential. In addition, all the photoconductive elements must be driven not only in a full-color mode but also in a monochrome mode, increasing parts cost and aggravating power consumption.
The problem with Laid-Open Publication No. 6-167858 also mentioned above is that the frequency of rotation variation must be increased because the rotation speed variation of each photoconductive element is effected by amplitude. While the frequency of rotation variation may be increased if the rotation speed of the output gear of the motor or drive source is noticeably increased, the increased frequency effects not only the photoconductive drums but also speed control over a sheet conveying system and image transferring mechanisms. Consequently, a period of time long enough for image formation is difficult to achieve, lowering the productivity of prints.
More specifically, as for the productivity of prints, assume that the rotation speed of the driveline is increased for the purpose of obviating irregularity in rotation between the photoconductive elements. Then, it is necessary to increase the operation speed of image transfer mechanisms for transfer ring toner images from the photoconductive elements and the operation speed of a sheet conveying system. This is apt to damage a sheet being conveyed or makes a conveying time necessary for fixation short. As for a fixing time, although a required fixing time may be guaranteed without regard to the increase in the rotation speed of the photoconductive elements, a plurality of conveying speed systems are necessary, one assigned to the time of conveyance via the photoconductive elements and the other assigned to the time of fixation, resulting in sophisticated control. Moreover, the irregularities of the individual gears are multiplied and make it difficult to reduce irregularity in rotation between the photoconductive elements even if the rotation speed is increased. Consequently, irregularity between the gears cannot be obviated unless the gears are machined with utmost accuracy, resulting in an increase in machining cost and therefore in the production cost of the entire apparatus.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 63-11965 and 4-54613, Japanese Patent Publication Nos. 7-31446, 8-14731, and Japanese Patent Laid-Open Publication Nos. 8-194361 and 2000-352851.